Available experimental data have been utilized to examine the effects of cross-coefficients on tracer diffusion and on the estimation of unidirectional fluxes from observations on tracer flow. In free solution or in a nonselective membrane, the interaction between the flows of tracer and the unlabelled substance are small at concentrations of biological interest for the nonelectrolytes urea, alanine, and beta-alanine, and for sodium and chloride ions. Under these conditions, measurement of tracer flow can be used to predict flow of the bulk substance to an accuracy of a few per cent.